Generally, electron emission devices can be classified into two types. In the first type, a hot (or thermoionic) cathode is used as an electron emission source. In the second type, a cold cathode is used as an electron emission source.
Known electron emission devices of the second type include a field emitter array (FEA) type, a surface conduction emitter (SCE) type, a metal-insulator-metal (MIM) type, a metal-insulator-semiconductor (MIS) type, and a ballistic electron surface emitting (BSE) type.
Although, electron emission devices differ in specific structure depending on their types, each electron emission device generally includes an electron emission unit contained within a vacuum vessel and an image display unit facing the electron emission unit in the vacuum vessel.
In an FEA type electron emission device, electrons are emitted from electron emission regions of an electron emission unit by electric fields formed when driving voltages are applied to driving electrodes located in the electron emission regions.
In the case that the FEA type electron emission device has a tri-electrodes system comprised of a cathode electrode, a gate electrode, and an anode electrode, it generally has a structure in which the cathode electrode is formed on a first substrate, a electron emission region is formed on the cathode electrode, and the gate electrode is formed over the cathode electrode interposing an insulating layer to expose the electron emission region. Each of the insulating layer and the gate electrode has a respective hole to expose the electron emission region.
However, in the electron emission device with the structure mentioned above, an electron beam emitted from the electron emission region through the gate electrode to the anode electrode does not always reach a desired phosphor layer and may be scattered around the gate electrode. As such, the electron beam may collide with an undesired phosphor layer to reduce color representation. To put it another way, in a conventional electron emission device, there is a problem in properly focusing the electron beam. To solve this problem, an electron emission device has been suggested which has an electrode for focusing the electron beam provided on a gate electrode, but the satisfactory focusing has not yet been achieved.